A conventional cathode structure for a cathode-ray tube, as shown in FIGS. 1A and 1B, comprises a cathode cap 13 coated with an electron-emissive substance 14 for emitting electrons when heated, a heater for heating the cathode cap, a cylindrical cathode sleeve 12 of Ni-Cr alloy for radiating the heat generated from the heater, a cathode sleeve holder 11 for holding the cathode sleeve 12, and a plurality of metal ribbons 16 for fixedly connecting the cathode sleeve 12 and cathode holder 11.
One end of the metal ribbons 16 is attached to the lower end of the cathode sleeve 12, and the other end to the upper surface of the cathode holder 11, so as to fix the cathode sleeve to the cathode sleeve holder. The reason that the metal ribbons 16 are employed to fix the cathode sleeve to the cathode sleeve holder is to prevent the heat transferred from the heater to the cathode sleeve 12 from being easily lost towards the cathode sleeve holder 11.
It is very important that the heat from the heater is transferred to the cathode cap 13 coated with the electron-emissive substance 14 without considerable heat lost, which reduces the power consumption of the heater, the time taken for the image to appear on the screen of the cathode-ray tube, and the temperature of the cathode sleeve holder and thus the current change of the cathode caused by the heat. Further, to this end, the cathode sleeve is blackened to improve the heat radiation. The cathode sleeve is made of Ni-Cr alloy, and blackened by oxidizing the Cr component in a wet hydrogen ambient. In addition, as shown in FIGS. 1A and 1B, metal ribbons are employed to fix the cathode sleeve to the cathode sleeve holder, thereby reducing the heat loss.
However, the following problems are caused by the metal ribbons of which one end is simply connected to the lower end of the cathode sleeve 12 and the other end to the upper surface of the cathode sleeve holder 11. Namely, the electric field formed around the cathode is concentrated on a plurality of projections 17 that are formed in the lower end portion of the cathode sleeve 12 by the metal ribbons 16, so that there may occur electric discharge between the heater 15 and the projections 17 to cut off the heater.